Water repellency effects on liquid- and vapor-phase water exchange in soil and clay minerals

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Drought conditions and wildfires can induce soil water repellency. Precipitation shifts are expected to exacerbate drought and wildfire in regions such as the southeastern United States, making it critical to understand how repellency affects water exchange processes in soil. The objectives of this dissertation were to 1) quantify the water vapor sorption dynamics of two clay minerals in which water repellency was induced; 2) identify if and for how long wildfires in humid hardwood forests induce water repellency, 3) evaluate if organic carbon content and hydrophobic functional groups explain actual and potential soil water repellency; and 4) understand how vertical position (i.e., depth) of water repellent layers affect infiltration processes. To meet these objectives, a laboratory test was first conducted examining water vapor sorption processes in water-repellent clay minerals. Next, a field study occurred in two forests that experienced wildfires in late 2016: Mount Pleasant Wildfire Refuge, Virginia, and Chimney Rock State Park, North Carolina, United States. Measurements include water drop penetration time, soil water content, and tension infiltration. Complimentary laboratory tests quantified potential soil water repellency, soil organic carbon content and hydrophobic functional groups. Results showed that water repellency inhibited water vapor condensation because of altered mineral surface potentials and decreased surface areas. Burned hardwood forest soils presented water repellency for > 1 year, though laboratory measurements presented different trends than in situ measurements. Total organic carbon content and hydrophobic functional groups correlated with soil water repellency measured in the laboratory but not the field. Soil water content was lower in burned than unburned soils, and negatively correlated with water repellency. Water repellency in the surface layers significantly reduced relative water infiltration rates, whereas subsurface water repellency did not, and water repellency persisted longer in sites with surface compared to subsurface water repellency. Finally, while the wildfires increased the occurrence of water repellency, they did not alter the underlying relationship between relative infiltration and surface water repellency. Altogether, this study provided new insight into water repellency effects on water partitioning at soil-atmosphere interfaces, and presented evidence of soil and hydrological changes induced by wildfires in humid hardwood forests.